This invention relates to a circuit arrangement suitable for operating at a supply voltage V.sub.n a high-pressure discharge lamp having a lamp voltage V.sub.la in conjunction with a current limiter circuit and with a first controlled semiconductor switching element provided with a control electrode connected to a control circuit of the circuit arrangement.
The terms "lamp voltage V.sub.la " and "supply voltage V.sub.n " are to be understood in this description and the claims to mean the value of the root of the time-averaged square of the value of the actual lamp voltage and supply voltage, respectively, i.e. the so-called R.M.S. value. The lamp voltage V.sub.la is the voltage across the lamp in the stable operating condition of the lamp.
The term "supply voltage V.sub.n " is also to be understood to mean in this description and the claims that voltage to which the combination of the current limiter circuit and the lamp is connected. The current limiter circuit may be constituted by a single stabilization ballast, such as a self-inductance. A semiconductor switching circuit, such as, for example, an up-converter or a down-converter, as the case may be in conjunction with a converter, is also possible.
A circuit arrangement of the kind mentioned in the opening paragraph is known from U.S. Pat. No. 4,475,065. In the known circuit arrangement, the first controlled semiconductor switching element is controlled so that during the lifetime of the lamp the power consumed by the lamp decreases in dependence upon the increase of the lamp voltage V.sub.la. Thus, it is possible for the lifetime of the lamp to be lengthened. Other parameters of the lamp, such as the general colour rendition index R.sub.a and the colour temperature T.sub.c of radiation emitted by the lamp, will vary under the influence of the known control. However, in many cases, for example in public illumination, these parameters play a substantially negligible part. This is quite different in those high-pressure sodium discharge lamps which are suitable for use as an interior illumination lamp, for example for illumination of sporting-halls or as a light source in a living-room. With the use as an interior illumination lamp, both the colour rendition index R.sub.a and the colour temperature T.sub.c are of great importance. It is therefore very important that these parameters remain constant or substantially constant during the life of the lamp. However, it is known that the colour temperature T.sub.c of the emitted radiation varies and more particularly decreases with the use of the known arrangement.